Cardiac arrhythmias are a major public health burden. There are 250,000 - 400,000 sudden cardiac deaths (SCD) in the US every year, accounting for up to 20% of all deaths in adults; the majority of these are due to ventricular fibrillation. In addition, approximately 2.5 million Americans now suffer atrial fibrillation (AF), a number that is expected to surpass 5 million by 2050. Strong evidence indicates that risk for both SCD and AF include a prominent genetic component. Genetically-modified mice are a powerful model to study mechanisms underlying cardiac arrhythmias and heart failure. In larger animals, clinical pacemakers can be used to develop models of chronic AF or pacing-induced heart failure, among others. However, it has not been possible to develop similar models in transgenic or knockout mice due to the lack of availability of miniature, implantable pacemakers. Existing implantable telemetry devices for cardiac monitoring in small animals are either too large, too heavy, or have inadequate battery life. Moreover, these devices are not able to pace the hearts, which is essential for the development of the above models. The goal of this contract is to develop a small (<1cc), light-weight (<1gram), batteryless, wireless programmable implantable device for in vivo stimulation and monitoring of cardiac electrical activity in mice and other small animals. With batteryless operation by resonant inductive power transfer and wireless operation with a standard mouse cage, such a device will be suitable for long-term, ambulatory pacing and ECG monitoring in mice.